Generally, lighting is an activity or function to light a certain place up using various light sources with a particular purpose. Lighting is mostly used to make an environment brighter in the night or in the dark.
FIG. 1 is a cross-sectional view showing a flat lighting device according to a prior art. Referring to FIG. 1, the lighting device according to a prior art includes a light source 10 and a louver or a reflecting shade 20. As for the light sources 10, incandescent light bulb, LED, CCFL, or the like may be used. Referring to FIG. 1, a light having an angle denoted with dotted line causes visually discomfort to a person when it is transferred to the person. Such a lighting device may reduce UGR mechanically, but cannot be a perfect flat lighting aesthetically.
FIG. 2 is a cross-sectional view showing a flat lighting device according to an embodiment of a prior art. Referring to FIG. 2, a lighting unit 30 includes a light source 10 and a diffusion plate 40 for diffusing a light emitted from the light source 10. The light emitted from the light source 10 is discharged outside through the diffusion plate 40. The diffusion plate 40 is used for reducing hot spot from the light source and emitting uniform light. Even if the diffusion plate 40 is used, as shown in FIG. 2, a light with the angle denoted with dots still render discomfort to eyes of a person. Namely, since the diffusion plate 40 scatters a beam up to the direction in which UGR is high enough causing fatigue of the eyes due to the glaring, so such a diffusion plate cannot conform with the standard of indoor flat lighting device. Accordingly, it is important to reduce the glaring to the eyes in indoor flat lighting. The extent of discomfort caused from the glaring to the eyes is represented using a constant called UGR (Unified Glare Rating). In other words, UGR is a value calculated by quantifying the extent of discomfort to the user of a lighting device.
UGR is calculated as a value of light flux emitting at the angle between 65 deg and 90 deg when a direction facing a bottom surface from a ceiling provided with a lighting device is set to 0 deg and a direction parallel to the ceiling is set to 90 deg. That is, the glaring to the eyes will reduce when the light flux at 65 deg-90 deg is reduced. In Europe and US, an indoor lighting device has to be below 19 of UGR.